1. Field of the Invention
The present invention relates to the bleaching of wood pulp. More specifically, the present invention relates to the flow control of chlorine as a wood pulp bleaching agent for the purpose of achieving a constant magnitude of pulp brightness.
2. Description of the Prior Art
Pursuant to the manufacture of paper and similar cellulosic products from wood, chips of the raw material are usually subjected to a thermochemical process known as digestion wherein the lignin binder of the natural cellulose fibers is, to varying degrees, dissolved, degraded and otherwise loosened from the fiber bond.
Although the dominant percentage of original lignin is removed in the digestion process, the resulting pulp is dark brown in color due to residual color bodies originating from the remaining lignin. If a white product is the objective use of the pulp such dark color bodies must be removed therefrom.
The prior art technique of removing color bodies from wood is characterized as the bleaching process which comprises several sequential steps of combining chemicals with a dilute aqueous slurry of the pulp followed by washing. Conventionally, the first of such sequential steps includes the addition of liquid or gaseous chlorine for the purpose of reacting with the residual lignin. The chlorine stage of the bleaching process is followed by the addition of caustic or sodium hydroxide for the purpose of dissolving and mobilizing the chlorinated lignin. Subsequently, the pulp may be treated with calcium or sodium hypochlorite for the purpose of further solubilizing remaining lignin compounds. Finally, the pulp is combined with chlorine dioxide which reacts primarily with the cellulose fibers to oxidize the surfaces thereof to impart a greater degree of reflectance, hence, whiteness, to the pulp.
The present invention is directed to the first or chlorine addition step of the bleaching process and the method and apparatus for controlling such chlorine additions.
Further to the prior art, washed pulp from the digestion step is mixed with chlorine and thereafter allowed a predetermined time of residence for the purpose of chemical reaction. The duration of such residence, which may be from 15 to 90 minutes, depends on the stock flow rate relative to the volumetric capacity of conduits and tankage between the chlorine addition point and the chlorine washers. Since in most pulp mills, the volumetric capacity is constant and the flow rate is variable for reasons independent of the bleaching process, the process may be controlled only by the chlorine addition flow rate.
Although a constant brightness or whiteness is the common objective of most bleach processes, the usual prior art control parameter of chlorine addition is that of chlorine residual. This term relates to the percentage of unreacted chlorine in the flowing pulp mass at a fixed distance downstream of the addition point. Chlorine residual is measured by electrically sensing the oxidation reduction potential of the aqueous solution which carries the pulp. Such a system is described by T. C. Burnett, Pulp And Paper Magazine Of Canada, Vol. 71, No. 14, July 17, 1970, pg. 57-62.
Chlorine residual may be related to pulp brightness only by operational history of a particular pulp mill and bleaching plant. The magnitude of chlorine residual at a fixed point in a flow stream required for a given degree of brightness at the end thereof would depend on the consistency of the water-pulp slurry, the relative quantity of lignin remaining in the dark pulp (a complex factor characterized as the pulp "K" number), the momentary stock flow rate and the stock temperature. All of these parameters are, in actual operation, highly variable, each being independent of the others. Therefore, a historical correlation between final brightness and chlorine residual may only achieve an average level of effectiveness. When the momentary stock conditions are such as to result in departure from the historical average, the pulp fibers are either weakened by the addition of excessive chlorine or, in the case of too little chlorine addition, additional bleaching burdens are thrust upon the subsequent bleaching steps. In such latter case, the cost/effectiveness of supplementary bleaching by the tertiary steps is poor as compared to that of chlorine treatment.
Although control of chlorine addition by the chlorine residual technique is generally understood by the industry as being the most economical approach, the usual cost equation considers only the chlorine cost relative to a given degree of brightness for typical pulp properties and flow conditions. When the cost equation is expanded to account for the quality loss due to over-chlorination or the diminished cost/effectiveness of tertiary step supplementation due to under chlorination, the singular factor of chemical cost significance dims considerably.
Notwithstanding such foregoing complexities, the first object of pulp bleaching is that of constant brightness and the most efficient known technique of achieving the objective, as a first step thereto, is by chlorine addition. Accordingly, sufficient chlorine should be added to the stock to accomplish the desired brightness regardless of chlorine residual at some fixed flow stream point. Pursuant to this objective, therefore, brightness should be the primary process control parameter.
An objective standard for measuring pulp brightness is as a percentage of reflectance from a white light source filtered to 457 millimicrons. In practice, however, such a standard is largely arbitrary since the human eye, perceptive of the entire white light spectrum, is the final judge of the quality.
Numerous instruments have been devised to measure the magnitude of white light reflectance from a surface such as that described by U.S. Pat. No. 3,771,877 to R. Rosencranz. Moreover, white light photosensors to control chlorine addition to a wood pulp bleaching flow stream were suggested by D. Noel Obenshain, TAPPI, Vol. 41, No. 1, January 1958, pg. 1. As in the case of residual chlorine measurement, however, the interacting independent variables of consistency, temperature and residence time strongly affect the reliability of such prior art photosensory chlorine control systems. In the case of the obenshain system, the difficulties were distributed among the facts of consistency, temperature and the long (15 to 90 minutes) residence time between chlorine addition and photo-sampling. This latter factor created an unreasonably long lag time between disturbance and detection in the feedback control loop.
Accordingly, it is the objective of the present invention to provide a photosensory bleach control system having a very short time delay in the feedback control loop but which is simultaneously insensitive to variations in consistency, residence time and temperature.
Another objective is to provide a simple photosensory apparatus for detecting slurried pulp brightness regardless of consistency variations.